It is common knowledge that the aircraft industry requires structures that on one hand withstand the loads to which they are subjected, meeting high requirements on strength and stiffness, and on the other hand are as light as possible. The main structure of the aircraft wing, usually referred to as the wing box, is particularly composed of skin and stringers. The skin is reinforced with stringers to reduce its thickness and so as to be competitive in weight
According to common knowledge of a skilled person, skin and stringers of a wing box may be made of metallic material or carbon fiber reinforced plastic.
It is also common knowledge that a wing box has to withstand to bending, torsion, and shear. Due to their geometry skins as thin panels are better suited to resist shear loads caused by lift and torsion, while stringers as beams are better suited to resist axial loads caused by the bending moment.
In case of metallic wing boxes, the amount of axial load absorbed by each structural component of the wing box, as the skin and stringers, depends only on the cross sectional area ratio by assuming that skin and stringers are out of the same material.
In case of composite wing boxes the amount of axial load depends also on the elastic material properties of each structural component. Therefore orienting the main structural stiffness directions of each component, skin and stringer, in the appropriate direction can also control the amount of axial load absorbed by each component. For example, by orienting all fibers of the skin in +45°-direction and −45°-direction with respect to the longitudinal axis of the wing box, the stringers absorb about 30% higher loads. The skins become axially unloaded and may serve mainly for shear loads.
All passive load alleviation techniques known so far are based on the nonalignment of the main stiffness direction with respect to the wing axis. Such nonalignment can be achieved by sweeping the stringers and/or skin fibers' orientation with respect to wing axis. Such approaches are able to increase the nose-down twist but they also cause a loss in bending stiffness, which requires reinforcements. Hence the weight benefit envisaged by the higher nose-down twist on outer wing vanishes at the end.
Therefore it appears better to keep the stringers parallel to wing axis and let them work for wing bending mainly, neglecting quasi the role of skins to contribute to wing bending stiffness. The skins shall serve for shear mainly, also taking the torsion of the wing box. By taking advantage of the anisotropy of the composite materials, the lay-ups of skins alone are used as design space through forward/rearward oriented fibers in order to generate the desired nose-down twist on outer wing.